(1) Field of the Invention
The present invention relates to an improved poly(p-phenyleneterephthalamide) (hereinafter referred to as "PPTA" for brevity) fiber. More particularly, it relates to an improved PPTA fiber having a very high tensile strength.
(2) Description of the Prior Art
It has been expected for many years that a fiber excellent in the tensile characteristics will be prepared from PPTA which is a rigid polymer. In fact, Kwolek (U.S. Pat. No. 3,819,587) and Blades (U.S. Pat. No. 3,869,429 and U.S. Pat. No. 3,869,430) proposed processes for preparing high-tenacity fibers from PPTA.
PPTA fibers disclosed in these prior art references and commercially available PPTA fibers such as Kevlar and Kevlar 49 are characterized by a high tensile strength, but in each of these PPTA fibers, the tensile strength of the single filament is 32 g/denier at highest. With recent progress in the industrial society, a demand for a fiber having a higher tensile strength is increasing. The reason is that a stronger material can be realized with reduced amount (that is, with a lighter weight and at a lower cost).
We made research with a view to satisfying this desire by enhancing the tensile strength in PPTA fibers, and we found that the tensile strength of the fiber, which is a fracture phenomenon value, is not determined by a single structural factor but various defect factors participate in the tensile strength in a complicated manner, and if at least one of these factors is not in the satisfactory state, a high tensile strength cannot be realized. It also was found that since known fibers have some defect factors or other, the monofilament tensile strength is 32 g/denier at highest and in order to realize a high tensile strength that cannot be at-tained in the conventional fibers, it is necessary to control all the defect factors below certain levels. It was further found that in order to obtain a fiber having such a high tensile strength, it is necessary to adopt considerably refined preparation conditions and pro-cedures. We furthered our research based on these findings, and as the result, we have now completed the present invention.